1. Field of the Invention
This invention relates to systems and methods for spectrophotometric measurement of biochemical compounds in the skin for non-invasive medical diagnosis and monitoring. Specifically, the present invention relates to the determination of the hematocrit or the absolute concentration of hemoglobin in the blood by multiple-wavelength optical plethysmography.
2. Discussion of Related Art
The total concentration of hemoglobin in blood (HbT) or the hematocrit (Hct), defined as the fraction or percentage of red cells in whole blood, are primary variables used by physicians to assess the health of a patient. The hematocrit is the fraction of the total blood volume occupied by the red blood cells, and hemoglobin is the principal active constituent of red blood cells. Approximately 34% of the red cell volume is occupied by hemoglobin. A value of HbT less than 10 g/dl or Hct  less than 0.30 indicates an anemic state which can impair the normal functions of the body. Severe anemia can lead to death when the quantity of hemoglobin becomes insufficient to supply oxygen to the brain and other vital organs. Patients with kidney disease, pregnant women, and young children in developing countries are especially susceptible to chronic anemia. Acute anemia resulting from loss of blood, infection, or autoimmune disorders can be life-threatening and requires close monitoring.
The conventional means employed to measure Hct in clinical medicine is to puncture the skin, draw blood from a vein or capillary into a small-diameter tube, and measure the solid (packed-cell) fraction that remains after centrifugation of the blood. Measurement of HbT in accordance with standard practice also requires drawing a blood sample, which is then subjected to a chemical or mechanical process to lyse the red cells and release the liquid hemoglobin. After transferring the hemoglobin to a cuvette, its concentration is measured either by direct spectrophotometry or by colorimetry following the addition of a chemical reagent.
Although a number of methods have been developed to make these sampling and processing steps less cumbersome, no device is yet available to physicians for the reliable and accurate measurement of Hct or HbT that obviates blood sampling.
A number of researchers and inventors have recognized the value of a completely noninvasive method for measurement of hematocrit or total hemoglobin concentration. Schmitt et al. (Proc. SPIE, 1992, Vol. 1641, pp. 150-161) adapted the principles of pulse oximetry to the noninvasive measurement of hematocrit of blood in intact skin. The method is based on the measurement of the ratios of the pulsatile (ac) and non-pulsatile (dc) components of the light transmitted through a blood-perfused tissue within two spectral bands in which the molar extinction coefficients of oxygenated hemoglobin (HbO2) and deoxygenated hemoglobin (Hb) are nearly the same. In one of the wavelength bands (800xe2x89xa6xcexxe2x89xa61000 nm), the absorption of hemoglobin is the dominate contributor to the attenuation of light in blood; in the other band (1200xe2x89xa6xcexxe2x89xa61550 nm), the absorption of water dominates. Therefore, the absorption of water serves as a measure of the plasma (non-cellular) fraction of the blood which does not contain hemoglobin. A hematocrit monitoring system based on a similar method has been disclosed by Steuer et al. (U.S. Pat. No. 5,499,627). In this disclosure, the influence of the optical properties of extravascular interstitial fluid on the accuracy of the measurement was recognized and the addition of a third wavelength was proposed to reduce this influence. The concept of adding more wavelengths to improve accuracy was extended further by Kuenster (U.S. Pat. No. 5,377,674) and Aoyagi et al. (U.S. Pat. No. 5,720,284). Steuer et al. (U.S. Pat. No. 5,499,627) also recognized the difficulty of obtaining a reliable plethysmographic pulse in the water absorption band (its amplitude is typically 4-10 times smaller than in the hemoglobin absorption band). To alleviate this problem, Steuer et al. (U.S. Pat. No. 5,499,627) proposed a method for inducing an artificial pulse by mechanical compression of the tissue at the location of hematocrit measurements.
In spite of these earlier advances, measuring the absolute concentration of hemoglobin in blood accurately and reliably remains difficult in practice. This difficulty stems mainly from two limitations.
The first limitation is the failure of the available mathematical algorithms used in the prior art devices to account for the fact that the blood vessels displace the extravascular tissue when they expand, because the tissue is essentially incompressible. Because of the incompressibility of tissue, the change in the diffuse transmission of light through tissue observed during arterial pulsation depends on the difference between the optical properties of the blood and the surrounding gelatinous tissue matrix. Therefore, to obtain an accurate measure of the absolute values of the hemoglobin concentration in the blood, one must also account for the optical properties of the tissue that surrounds the blood vessels in the skin. Measurement of the ac/dc ratios alone, regardless of the number of wavelengths at which the measurement is made, cannot compensate entirely for the variations in the scattering and absorption properties of the skin of different subjects. This problem is not important in conventional pulse oximetry because the attenuation of light in blood greatly exceeds that in the surrounding tissue at the wavelengths at which ac/dc ratios are measured (typically 660 nm and 910 nm). The same is not true in the measurement of HbT by optical plethysmography, however, which relies on the measurement of pulsations resulting from optical absorption of water in the blood. Because the volume fraction of water in blood is close to that of the extravascular tissue matrix, the difference between the absorptivities of blood and the surrounding tissue is small within water absorption bands. Moreover, the difference between the scattering properties of blood and the surrounding tissue vary with their relative water contents. Accordingly, one limitation of the prior art devices and methods used to noninvasively measure hematocrit or hemoglobin has been the inaccurate measurement of tissue water.
The second limitation is the reliance of the prior art methods on the measurement of small pulsatile changes in the blood volume induced by contractions of the heart. When the water contents of the blood and the extravascular tissues are nearly the same, the pulsatile (ac) component of intensities measured at wavelengths greater than 1250 nm are usually less than one percent of the mean (dc) intensity. Even using the most advanced circuitry and signal-processing techniques, the amplitudes of such small pulsations are difficult to measure reliably. Although mechanical compression of the tissue, as proposed by Steuer et al. (U.S. Pat. No. 5,499,627), alleviates this problem by inducing a larger blood volume change, it also introduces large changes in the scattering coefficient of the bulk tissue which can complicate calibration of instruments based on this technique, because the compression is occurring at the same location as where the hematocrit measurements are taken.
Therefore, there exist a need for more reliable and accurate measurement of hematocrit by noninvasive means.
The objective of embodiments of the present invention is to provide a more reliable and accurate measurement of hematocrit (Hct) by noninvasive means. The changes in the intensities of light of multiple wavelengths transmitted through or reflected light from a tissue location are recorded immediately before and after occluding the flow of venous blood from the tissue location with an occlusion device positioned near the tissue location. As the venous return stops and the incoming arterial blood expands the blood vessels, the light intensities measured within a particular band of near-infrared wavelengths decrease in proportion to the volume of hemoglobin in the tissue location; those intensities measured within a separate band of wavelengths in which water absorbs respond to the difference between the water fractions within the blood and the displaced tissue volume. A mathematical algorithm applied to the time-varying intensities yields a quantitative estimate of the absolute concentration of hemoglobin in the blood. To compensate for the effect of the unknown fraction of water in the extravascular tissue on the Hct measurement, the tissue water fraction is determined before the occlusion cycle begins by measuring the diffuse transmittance or reflectance spectra of the tissue at selected wavelengths.
An important feature of the embodiments of this invention is that it incorporates a means of compensating for natural variations in the water fraction of skin of different individuals. Such variations affect both the scattering and absorption coefficients of skin in the near-infrared region of the spectrum and are a primary source of error in hematocrit estimates derived from the assumption that the optical coefficients of the extravascular tissue are fixed quantities.
An equally important feature of the embodiments of this invention is that the relatively large change in blood volume induced by the venous occlusion facilitates the measurement of small differences between the optical properties of the blood and the extravascular tissue. Thus, better accuracy can be obtained compared to methods that rely on arterial blood pulsations. It should be understood, however, that the mathematical algorithm on which the present invention is based applies equally well to intensity changes induced by natural arterial pulsations or compression of the skin.
For a fuller understanding of the nature and advantages of the embodiments of the present invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings.